1. Field of the Invention
This invention relates to brazing of metal parts and, in particular, to a homogeneous, ductile nickel-based brazing material useful in brazing stainless steels and high nickel alloys.
2. Description of the Prior Art
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, corrosion resistant, leak-tight joint is formed.
The selection of a particular brazing filler metal for a specific application depends basically on its solidus and liquidus temperatures. The term "solidus" refers to the highest temperature at which a metal or alloy is completely solid, and the term "liquidus" refers to the lowest temperature at which the metal or alloy is completely liquid. In any brazing process, the brazing filler metal must possess a solidus temperature that is high enough to provide it with adequate strength to meet the service requirements of the metal parts brazed together. Certain critical parts of aircraft engines, such as honeycomb air seals, are in service at high temperatures. Therefore, the filler metals used for brazing these parts have high solidus temperatures. A filler metal designated as AWS-BNi-5 is commonly used in such applications because it possesses the highest solidus temperature in the entire BNi family of alloys. This filter metal also provides adequate strength and corrosion resistance due to its high chromium content (about 19 wt. percent).
A major drawback of the AWS BNi-5 composition is the difficulty of producing it in ductile foil form. The AWS BNi-5 composition contains a substantial amount (about 10 wt. percent) of the metalloid element silicon and has heretofore been available only in the form of a powder, powder-binder paste or powder-binder tape. Powders are generally unsuitable for many brazing operations such as brazing parts of complex shapes. The organic binder present in powder-binder paste and powder-binder tapes bakes off during brazing, leaving objectionable voids and residues which are detrimental to the joints. A 100% dense, flexible brazing filler metal foil having characteristics similar to BNi-5 would, therefore, be highly desirable for many brazing applications.
Some brazing alloys are available in foil form. Such materials are (1) fabricated through a costly sequence of rolling and careful heat-treating steps, or (2) fabricated by quenching a melt of the alloy on a rotating quench wheel at a rate of at least about 10.sup.5 .degree. C./sec. Rolled foil is not sufficiently ductile to permit stamping of complex shapes therefrom. Quenched foil, disclosed by U.S. Pat. No. 4,148,973, represents a substantial improvement over rolled foils, but has heretofore evidenced melting, oxidation and corrosion characteristics not especially suited for effecting high temperature brazements, such as honeycomb air seals in aircraft turbine engines.
Ductile glassy metal alloys have been disclosed in U.S. Pat. No. 3,856,513, issued Dec. 24, 1974 to H. S. Chen et al. These alloys include compositions having the formula M.sub.a Y.sub.b Z.sub.c, where M is a metal selected from the group consisting of iron, nickel, cobalt, vanadium and chromium; Y is an element selected from the group consisting of phosphorus, boron and carbon; and Z is an element selected from the group consisting of aluminum, silicon, tin, germanium, indium, antimony and beryllium; "a" ranges from about 60 to 90 atom percent; "b" ranges from about 10 to 30 atom percent. Also disclosed are glassy wires having the formula T.sub.i X.sub.j, where T is at least one transition metal and X is an element selected from the group consisting of phosphorus, boron, carbon, aluminum, silicon, tin, germanium, indium, beryllium and antimony; "i" ranges from about 70 to 87 atom pecent and "j" ranges from about 13 to 30 atom percent. Such materials are conveniently prepared by rapid quenching from the melt using processing techniques that are now well-known in the art. No brazing compositions are disclosed therein, however.
There remains a need in the art for a brazing alloy that has characteristics (i.e., high solidus temperature and superior resistance against corrosion and oxidation) similar to BNi-5, but is readily manufactured in the form of a thin, homogeneous, ductile foil.